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During a normal Apollo mission, the command/service module was undocked from the lunar module using the reaction control system engines on the service module. (Although the command module had some RCS engines, they were only for attitude control, not to provide translation.) However, during Apollo 13, the service module was already jettisoned before the lunar module was undocked. How were the CM and LM able to safely separate without the help of the SM?

This question is part of a series honoring the 50th anniversary of Apollo 13, "NASA's finest hour".

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According to this article:

The docking tunnel connecting the two ships was pressurized, though, so the idea was to use that pressure as a kind of “air spring” to push the two spaceships apart.

and later:

NASA wanted just one number: What should the pressure in the docking tunnel be at separation?

Too much, and the astronauts’ hatch might be fatally damaged.

Too little, and the lunar module wouldn’t be pushed far enough away, and could also endanger the astronauts.

And more:

By the appointed time, they had agreed on a number: 2 psi in the tunnel, a level the astronauts would set. That was much lower than the standard 5 psi, but enough to push the lunar module away, as Sullivan recalls it, at 2 feet a second, about 1.5 mph.

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